This invention relates to the monitoring of the pressure of the tires of a vehicle.
The operating life of a vehicle tire which is under or over inflated can be significantly reduced, and this has significant financial implications when one considers the cost of tires, especially for large road vehicles and for aircraft. Apart from this there is the safety aspect for the handling characteristics of the vehicle, or aircraft, with underinflated tires deteriorate and, moreover, the operating costs under such circumstances is increased because of the increase in rolling friction.
Many solutions have been proposed in the patent literature to the problem and such solutions are exemplified in the specifications of the following patents: U.S. Pat. Nos. 2,727,721, 3,016,515, 3,588,815, 3,713,092, 3,805,229, 3,938,077, 3,950,726, 4,037,192, 4,048,614, 4,210,898, 4,059,823, 2,248,047, 3,281,784, 3,697,947, 3,757,295, 3,662,335, 3,723,966, 4,006,449, 4,208,649, 4,237,445, 3,155,938 and 3,249,916, and in German Pat. No. 942904.
Broadly speaking the solutions fall into two categories. In the first category use is made of radio techniques for transmitting a signal from the tire to the vehicle while in the second category use is made of hard wire techniques or static devices i.e. devices which are not electrically powered to provide an indication of the tire pressure.
The first category of devices can make use of transponders but normally active transmitters are built into each wheel and consequently there is the need for a self-contained power source in the wheel. With the second category of devices and referring particularly to the hard wire device power is supplied to pressure sensing means in the wheel from an external power source. With this kind of device the question of signal processing is of critical importance.
Of the aforementioned patent specifications four warrant closer examination. In U.S. Pat. No. 3,281,784 use is made of a bridge circuit and a galvanometer which proves an indication of pressure increase or decrease. This device suffers from the disadvantage that it requires an initial calibration and moreover the readout is not directly proportional to the tire pressure. In U.S. Pat. No. 4,237,445 use is made of a Bourdon tube which communicates with internal tire pressure to regulate a magnetic gap of an electromagnetic induction configuration. U.S. Pat. No. 2,248,047 discloses a mechanical switching device which is mounted externally on the wheel and which employs two slip ring assemblies for conveying a signal to the interior of a vehicle. German Pat. No. 942904 discloses an analogue system wherein the current through a device, the resistance of which is determined by tire pressure, is monitored.
In all of the prior art devices there are various shortcomings. Generally it may be said that these shortcomings relate to the difficulty of installing the tire pressure monitor on the wheel in question and secondly to the manner in which the signal is conveyed from the tire to inside the vehicle. Intimately associated therewith is the question of powering the pressure sensing device.